Recent developments in the art of manufacturing glass fibers have resulted in new methods for making irregularly shaped glass fibers comprising two distinct glass compositions with different coefficients of thermal expansion. The difference in coefficient of thermal expansion between the two glasses enables the resulting glass fiber to have an irregular shape, thereby providing numerous attributes to the glass fiber which enhance the performance of products made with those fibers. Some of those product enhancements include increased entanglement among the fibers, increased recovery of insulation products made by the fibers, lower K values, lower irritability upon handling, and a more uniform volume filling nature.
Dual-glass fibers disclosed in the art so far were comprised of glasses that have generally similar viscosities and softening points. For example, in U.S. Pat. No. 2,998,620, to Stalego, the differences in viscosities and softening points of the four glass compositions disclosed are relatively minor. Since the viscosities and softening points of the two glasses of the dual-glass fibers of the prior art are relatively similar, the two glasses will react similarly to the fiber forming conditions encountered by the dual-glass stream as it forms into a solid dual-glass fiber. It would be advantageous to make a dual-glass fiber from a molten dual-glass stream where there are differences in the viscosity and/or the softening point of the two glasses in order to enable the two glasses to exhibit differing reactions to the fiber forming environment.